1. Field of the Invention
The present invention relates to power detectors, and more particularly to a power detector using a mirror cell.
2. Description of the Prior Art
A power amplifier is a type of electronic circuit that converts a low-power signal into a signal of higher power. The power amplifier is typically applied as a last amplifier in a transmission circuit. For example, a mobile phone antenna may be driven by a power amplifier for emitting a radio frequency (RF) call signal at a power level sufficient for communicating with a base station located anywhere from 400 yards away to as far as 20 or more miles away.
In order to ensure transmission of a clean, linear signal at an appropriate power level, while maintaining power efficiency, a power detector is typically coupled to the power amplifier for detecting output power of the power amplifier. The power detector is generally coupled directly to a node of the power amplifier located between the output of a final amplification stage of the power amplifier and the input of an antenna. The power detector is usually coupled in front of an output matching circuit, but may also be coupled after the output matching circuit.
In the configuration described above, the power detector causes many design problems. First, performance of the output amplification stage is affected by the power detector. Output-matching, gain optimization, and linearity optimization become significantly complicated. Second, performance of the power detector is affected by the output load of the final amplification stage. In modern communications systems, power detection should be independent of system or environmental voltage standing wave ratio (VSWR). However, the power detector senses both the amplified output signal and a reflected signal from external circuitry. Thus, the power detector output is very sensitive to output-loading variations and/or impedance mismatch between the final amplification stage and the external circuitry, which induces significant dependence on system VSWR.